--> Abstract: Pinch-Out Style And Tidal Influence Of A Regressive Transgressive Wave-Dominated Deltaic Sandbody, 20-Mile Sandstone, Mesaverde Group, Nw Colorado, by L. Seidler and T. Benda; #90928 (1999).

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SEIDLER, LARS1 and TARA BENDA2
1Dept. of Geology, University of Copenhagen, Denmark
2Dept. of Geology, University of Wyoming, USA

Abstract: Pinch-Out Style and Tidal Influence of a Regressive Transgressive Wave-Dominated Deltaic Sandbody, 20-Mile Sandstone, Mesaverde Group, NW Colorado

The Upper Cretaceous 20-Mile Sandstone in NW Colorado is excellently exposed and the three-dimensional geometry and facies variations reveal pinch-out style, lateral continuity and depositional processes responsible for this shallow marine sandbody. In its proximal reaches the sandstone is 40 m thick without shale interbeds. The lower two-thirds of the body shows a clinoform set dipping at an angle of 2-3<deg>. Within 2-3 kilometers the sand-rich body splits up towards north and east into four tongues separated by shale interbeds. The bi-directional split-up indicates that the coastline was irregular and defined by several delta lobes. The relative pinch-out distances of each sandstone tongue together with the varying thickness of the intervening shale units highlight the regressive-transgressive character of the body. Tidal influence was apparently strongest during deposition of the most regressive sandstone tongue, i.e. in the highstand systems tract. This may indicate that the palaeo-transgression was very low and did not enhance tidal currents during transgression, contrary to what has been reported elsewhere.

The uppermost sandstone tongue, i.e. that in the transgressive systems tract, has a geometry different from the underlying tongues. The frequency of interbedded shales is much greater, reflecting a higher accommodation to sediment supply ratio, and the lateral continuity of sands is much less that in the underlying regressive tract.

Changes in the relief of the deltaic coastline, irregular rates of overall relative sea level rise, and abundant sediment supply appear to have been the main controls on the development of this latest stage of the 20-mile sandstone.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas